PPT Dollar Drop
Version Date Who Revisions
04 8/6/03 sc Deleted references to graph of varied finger distances
Added explanation of opening activity- do as POE
Added explanation of exploratory activity & discussion
Revised student pages to reflect exploration
05 2003/ dk added reading and exercises
06 2004/ Sc Edited to fit new sequence
06/28 Reading is now also included in Electric cars, I don’t
know where it best belongs.
DK - This still needs improvement in the teacher's section on the goals of the activity and
teaching experimental controls, variables, what to measure, etc.
Review linear motion, acceleration, acceleration due to gravity
Introduce physiology of reaction time
Have students develop their own experiments
Concepts & Skills
Science Experimental controls
A few class periods
This activity is a good review of previous concepts, like acceleration and gravity, that student’s
need to recognize and remember in order to solve the problem. We are using what we have
already learned in physics to explore physiology! From this activity, we can further explore the
nervous system, and the physics involved with how the nervous system works. The students also
have a chance to design their own experiments completely from scratch!
To start the activity and get the students interested, take out a dollar bill and demonstrate
dropping it and catching it with your index finger and thumb (it’s easy for you to catch it
yourself). Then ask for a volunteer. Tell your volunteer that if he or she can catch the dollar
then he or she can keep it. Hold the dollar bill between the volunteer’s index finger and thumb
with the face of the dollar even with the fingers. Make sure the dollar is not touching the fingers.
Have the rest of the class predict if the volunteer will catch it or not (do a POE). Most likely
your volunteer will not catch the dollar (however some of these young kids have really quick
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reaction times!). The rest of the class can start to think about why he or she could not (or could)
catch the dollar.
Ask the class, “How could you measure how long it takes to catch something dropped, like the
dollar bill.” Have them work in groups to brainstorm ideas and design an experiment using any
materials you make available (such as meter sticks, rulers, index cards, stopwatches, etc). After
the groups run their experiments, have a class discussion about each group’s data and
experimental design. Discuss sources of error and how the groups can compare results. This
should lead to the students realizing the need for certain controls and a set procedure for the
entire class. Scientists need consistent, repeatable results that they can compare.
It's hard for the students to give up stopwatches. Have a few students try to start and stop the
stopwatch as fast as they can and compare the times they get to the times they found on their
original tests. The numbers will be similar, so were they timing the catch or how fast they can
turn the stopwatch on and off?
Have the class work on developing an improved design, leading them, if necessary, to dropping a
ruler. A ruler is heavy enough to fall straight, and it has built-in markings. They can drop the
ruler and measure the distance it fell before it was caught Hopefully, the students will remember
that the ruler is accelerating and that they can use the d = 1/2 at2 formula to calculate the time it
took the ruler to fall.
Important controls that should come out of the discussion include:
finger spacing should be consistent
the ruler should always start at the same point
the ruler should be dropped straight
the catcher shouldn't know when the ruler will be dropped
the catcher's hand should be constrained so it doesn't move
there should be more than one drop/catch for each student (3? 5?)
the units used for measurement should be consistent
There may be other ideas:
the catcher should use their dominant hand
the windows should be closed to stop wind
everyone should use the same type of ruler
One of the points of this exercise is how difficult it is to control even a simple experiment, like
dropping and catching a ruler - there are a lot of factors to consider. Scientists need to think
about controls, sources of error, and what they can measure whenever they are designing or
running an experiment.
When all the groups have finished collecting data using the ruler, compare the data as a class.
Why are there variations in the data? Is it because of the falling ruler? Or is it because of the
person catching it? From physics, we know that all objects on Earth have the same acceleration
due to gravity, so it can’t be the ruler! It must be the person catching it! What is involved
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Note: The following student activity pages do not have to be handed out to the students.
Embedded assessment: Distance measurement and data recording.
Can the students read a centimeter ruler?
Do they remember to use SI units and not English units?
Are the appropriate units used and recorded?
Answers to Exercises
Answers to Challenge/ extension
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How could you measure how long it takes to catch something dropped, like the dollar bill?
Anything available from your teacher (like meter sticks, stopwatches, paper, index cards, rulers,
paper clips, string, etc.).
1. Write out the steps that you will take to conduct your experiment.
2. Conduct your experiment and record your data.
Some questions to keep in mind:
How many times should I repeat the data collection?
What are some important things to keep the same (control) in each trial?
What is a good way to organize the data?
1. Can you easily compare your data with another group’s data?
2. How can each group collect data that is easy to compare?
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What is science? It's many things. Mostly, it is the search for patterns in the natural world.
Science requires people working, sharing ideas, and building on each other's knowledge. Here's a
list of what science is:
Communication (including a common language)
Accumulated knowledge (what we know)
Common methods (how we work)
When people find a pattern in something, they think they understand it. Better yet, if they can
predict what will happen in the future, they really think they understand it. The test of knowledge
in science is experiment. Scientists test their ideas. If an idea is not testable, it is not scientific.
So scientists make predictions and test them to see how well they understand. A prediction may
turn out to be right, but just being right doesn't prove you understand it - you might be right for
the wrong reason or maybe it was a lucky guess. A testable (scientific) idea is one that can be
proved wrong. In Chicago, the amount of ice cream sold and the number of murders always
increase and decrease together. Does eating ice cream lead to violence? No, they both increase
during hot weather. As Einstein said, "No number of experiments can prove me right; a single
experiment can prove me wrong."
What makes a good experiment? A good experiment is as simple as possible, so other people can
understand and repeat it. Sources of error are minimized. In the simplest experiment, everything
stays the same except for one thing that is changed on purpose. Then, if other things change, it is
probably because of the one thing that was changed. The things kept the same are called controls.
The one thing that is changed is the independent variable. The thing that changes is the
dependent variable. If you drop a ball from different heights to see how high it bounces, the
height from which you drop it is the independent variable, and how high it bounces is the
dependent variable. Using the same kind of ball and the same kind of floor are controls.
In order for scientists to communicate, they need to share a common language. Latin was the
language of science, but the most common language used today is English. There are many
words with special definitions that scientists use so they can be clear with each other. Scientists
also use a lot of mathematics. Incredibly, the natural world can often be described
mathematically! It is one of the great mysteries of the world - for example, why would a ball
flying through the air be describable by an equation (almost) as simple as y = x2?
Scientists prefer to express their findings in numbers, or quantitatively, because numbers are
easy to compare and manipulate, but not everything can be expressed in numbers. Many things
are better described in words, or qualitatively. Most of the time scientists use both qualitative and
quantitative descriptions. Describing a fruit as round and orange is qualitative. Describing it as
having a diameter of 0.08 m and a mass of 0.3 kg is quantitative.
Scientists also need to use consistent measurements so they can compare their data. Long ago,
people used measurements like foot, hand, span, and cubit to measure distance (the height of a
horse is still measured in hands). Those measurements are all taken from the human body, but
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they vary from person to person. Now we use the metric system, which uses meters, kilograms
and seconds (mks).
One other very important aspect of science is honesty. Scientists must be honest. If they aren't,
they don't contribute to the growth of scientific knowledge, and they can harm people. In fact,
scientists are always verifying each other's work. If a scientist ever lies and is found out, they
will never be able to work as a scientist again, because no one will trust them.
Physics is the most basic science, studying what matter or "stuff" is, and how it interacts with
other matter. Originally, physics was the name for all of natural science (as opposed to
metaphysics, the study of things beyond physics, like philosophy and religion). As the
knowledge in science has grown, parts of physics have become their own fields, like chemistry,
biology, and physiology.
Identify the independent and dependent variables in these scenarios.
1. Plants in the sunlight grow taller than those in the shade.
2. Dogs pant more when it is hot outside.
3. Some cows eat grass and some eat corn. Cows that eat corn have a higher fat content than
cows that eat grass.
4. Fish often will move into an area where other fish are present, but not into an area where they
don’t see other fish.
5. In years when there is not very much to eat, birds will lay fewer eggs than usual.
6. In one research study, more mice got cancer when they were fed a certain chemical than mice
that did were not fed the chemical.
7. In one research study, more worms died when they were kept in still water than when they
were kept in running water.
8. In one research study, fish eggs exposed to caffeine hatched earlier than fish eggs that were
not exposed to caffeine.
Read each of the following descriptions of scientific studies. Underline the parts of the study that
you feel may cause error. Describe how you would change the study to fix that problem.
9. Dr. Jones studied cows. He wanted to know if cows gain more weight on a diet of grass or a
diet of soybeans. He put twenty cows in each group. The cows in the grass group were all
males and the cows in the soybean group were all females. He fed each cow two pounds of
food per day. He weighed the cows for the first time after they had been eating the special
diets for a month. He weighed them once a month for six months after that. At the end of the
study, he concluded that cows that eat grass gain more weight.
10. Dr. Martinez studied bats. She wanted to know if bats prefer to eat apples or plums. She cut
the fruit into pieces. She wanted to put an equal amount of fruit in each dish, but plums are
smaller, so she cut the plums into 1 centimeter pieces and the apples into 3 centimeter pieces.
She left the fruit out in dishes overnight in the nature preserve and counted the fruit pieces
that were left in each dish the next morning. She concluded that bats prefer plums because
there were fewer pieces of plum left in the dish.
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11. Dr. Wong studied worms. He wanted to know if worms living in sand grow longer than
worms living in mud. He employed two graduate students, Jason and Marla, to measure the
worms. Jason measured the worms in the sand, and Marla measured the worms in the mud.
Because Marla’s measurements were on average a little longer than Jason’s, Dr. Wong
concluded that worms grow longer when they live in mud.
12. Dr. Francis studied tiger sharks. She wanted to know if male sharks have longer teeth than
female sharks. She caught 5 male sharks and 5 female sharks, and measured their body
length and their teeth. The male sharks were between 3 and 3.5 meters long and the female
sharks were between 3.5 and 4.5 meters long. Overall, the male sharks had teeth that were
between 5 and 6 centimeters and the females had teeth that were between 6 and 8
centimeters. Dr. Francis concluded that female sharks have longer teeth than male sharks.
13. Dr. Smith studied orchid plants. She wanted to know if one species of orchid has more
chlorophyll (the green pigment in plant leaves) than another. To save time she ran her
pigment tests at the same time using two different spectrometers (machines that test the
amount of pigment). Her test showed that orchid species A has slightly more chlorophyll
than orchid species B.
Make a list of 6 things that you think might influence how quickly an individual can react to an
event such as catching a falling object. Explain why you think these factors would positively or
negatively influence how quickly someone can catch a dropped object.
Control - in an experiment, something that is kept the same
Independent variable - in an experiment, something that is changed intentionally
Dependent variable - in an experiment, something that changes in response to an independent
Quantitative - expressed in numbers
Qualitative - expressed in words
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